The gas turbine engine is the preferred class of internal combustion engine for many high power applications. Fundamentally, the gas turbine engine features an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between. The combustion chamber can include multiple mixers to mix fuel and air prior to combustion. Typically, each mixer requires an individual fuel injector tip (nozzle) to spray the fuel into the mixer's airflow.
In some operating conditions on a turbine engine, multiple fuel injectors are tuned to flow equal amounts of fuel. Reducing injector-to-injector variation enables better control of local fuel-to-air ratios in the combustor and allows for uniform temperature distribution at the inlet to the turbine stage. Uniform fuel flow from injector-to-injector is accomplished by calibrating the nozzle's flow number (FN) at a key operating condition. The pressure drop of the nozzle is adjusted to be in a narrow range at a known flow. In some applications, the calibration point tends to be at a high flow, where the accuracy of flow from injector to injector is most critical.
Nozzle calibration can be accomplished by adjusting the pressure drop of a flow restriction in the nozzle, which is typically in series with the tip restriction of the nozzle. This adjustable flow restriction in the nozzle is referred to as the calibration orifice. Nozzle flow typically passes through the calibration orifice and then the tip, which combine to give the nozzle its high-flow pressure drop characteristic.